At present, in the coating of paper or board, two alternative methods and devices are commonly used, i.e. a blade coater or a bar coater. The present invention is expressly related to the latter, bar coaters, which have proved excellent especially in the film size press technique. The material of the coating bars currently in use is usually steel, and, in view of increasing the service life of the bar, the bars are provided with chromium plating. In surface sizing of paper, bars with fully smooth faces have not been used. Rather, the face of the coating bar has been provided with grooves, or alternatively steel wire is wound onto the bar to form a solution similar to grooves on the bar face. A grooved bar is used for the surface sizing of paper because the thickness of the size film to be applied onto the base to be coated is determined by means of the depth of the grooves.
A grooved bar is, however, poorly suitable for the preparation of thin size films, because it is very difficult to manufacture grooves of sufficiently small scale. On the other hand, such small grooves are easily contaminated. It is a further highly significant drawback of grooved bars that they are worn rapidly. Pigmenting with a high dry solids content is also entirely impossible with grooved bars, because the wear of the bars is excessive in this connection. The diameters of the coating bars currently in use have been very small. Small-diameter bars are used because the bar should be able to adapt itself to the shape of the roll face in the size press in order that the profile of the size film could be made suitable and correct. The order of magnitude of the diameter of currently used coating bars is preferably about 10 mm.
With respect to the prior art, reference is made in particular to the U.S. Pat. No. 2,097,564, from which a solution is known for spreading of a size film or of a pigment coating film onto the rolls in a size press, said films being transferred to the paper in the nip in the film size press. In the method described therein, smooth or grooved bars are used for the metering of the size film. It is stated therein that while it is possible to use a smooth-faced bar of small diameter (diameter about 9 mm) for surface sizing, such a bar is unsatisfactory. Accordingly, a corresponding solution has never been applied in practice.
In respect to the prior art, reference is further made to FI Patent No. 30,147 and to applicant's FI Patent Appl. No. 901967 of earlier date.
From applicant's FI Patent Appl. No. 901967, it is known to control the quantity of size and/or pigment coating in respect of the profiles by means of a large-diameter bar by using special constructions either in the bar itself or in the roll. The quantity can be controlled by using a special construction in the roll whose face is coated and from which the coating is transferred to the paper in the nip in the size press.
In this earlier patent application, it should be noted that the bar comprises a small-diameter body on which bushings have been fitted. The bushings are attached to one another in a suitable way as non-revolving. With regard to special constructions related to the roll, rolls are provided with coatings of particular cross-sectional shapes and, in addition, rolls in a size press in which each of the rolls in the nip is a variable-crown roll.